The Ames Limestone is the boundary between the Casselman and Glenshaw Formations within the Conemaugh Group. It is a stratum that contains what many call the best fossils in Western Pennsylvania. The Upper Freeport coal bed is the boundary between the Allegheny Group and Glenshaw Formation. Therefore in Armstrong County, the boundary between the Allegheny and Glenshaw can be found along the rivers.
From top to bottom, it goes:
The Ames marks the boundary between the younger Gzhelian, the last stage in the Upper Pennsylvanian, and the older Kasimovian. While many older papers reference names such as Missourian or Virgilian Stages, the International Commission on Stratigraphy uses standard global names.
|Global Stage||Age||Time Represented|
|Gzhelian||4.8 MY||298.9 – 303.7 MYA|
|Kasimovian||3.3 MY||303.7 – 307.0 MYA|
On the following map, the Ames is shown as the yellow to orange merge points. Published in 1980, this map is by the Commonwealth of Pennsylvania Department of Environmental Resources. One interesting detail bout this map is that the freeway portion of PA 28 terminates shortly after passing the Freeport area. This shows the age of the map, published about 40 years ago.
The Ames preserves fossils dating back to the Carboniferous Period. The best local marine fossils are supposed to be in this layer.
One of my goals in local exploration is to find sources for Ames limestone. So far I have found none, but according to the maps, there are many places to search.
Here is a map of the Casselman Formation embedded on Google Maps.
In a 1933 report by H.H. Hughes, he reported the Ames Limestone to be nearly 30 inches thick near Perrysville, which I believe was in Bell Township, South East of Vandergrift. In the Plum, Lower Burrell area it was 16-35 inches thick, and completely absent at two drill sites.
Fragments found in the Northern part of the Freeport Quadrangle, South Buffalo Township showed thicknesses of 3 to 4 inches.
Compared to the Brush Creek Limestone
The Brush Creek Limestone is a much darker matrix, likely due to more anoxic environment when the sea invaded the land. A lack of oxygen means more of the biomass from the living creatures above doesn’t break down at the bottom of the sea. The Brush Creek is an older limestone, with a separation of approximately 2.3 million years of time between the two ancient seas.
Exploring Ames Limestone
The first place I got to see the limestone up close was in Frick Park. While there are several locations in which to find it, I have yet to find one with exposed rocks. I’ve done online searches for Ames Limestone and have often landed on the Sedimentation in Western Pennsylvania page, an old Pitt Geology web resource.
Following their directions, I quickly found two sets of outcrops. I could not find the hard ledge in the stream bed as mentioned. However, it appears there has been a lot of erosion in the stream bed, and there were several blocks of either sandstone or limestone along the creek.
I did not bring a hammer or chisel. While reading online, I quickly found out that you are not allowed to be busting up rocks in Frick Park. However, it was nice to finally see it up close.
Ames Limestone in Gilpin Township
The limestone cut above is a few miles from one of the termination Northern fingers of Ames limestone that stretch North East along the rolling hills of the Allegheny Plateau. The farthest Northern mapped portion of the Ames exists a bit past DuBois, PA in Penfield, PA. This is over 60 miles from Parks Township.
While the Casselman layer may be eroded away, it does not necessarily mean the Ames Limestone is gone. The stone is likely to remain long after the base layer and more is eroded. I often find what is called float limestone in local streams, still around after the more erodible layers have been washed away.
Ames Limestone along Route 28.
There are several places the Ames outcrops along Pennsylvania Route 28. During construction of the road, massive amounts of landscape was removed, exposing many hidden rocks. It can be seen more or less along stretches from Etna to Tarentum.
More Information about the Ames
- Paleoenvironments of the Upper Pennsylvanian Ames Limestone and associated rocks hear Pittsburgh, Pennsylvania – GeoScienceWorld [Subscription Required]
- Pennsylvania and Ohio Shape Files
H.H. Hughes (1936) – Topographic and Geologic Atlas of Pennsylvania No. 36 Freeport Quadrangle [54mb zip]